Concerns about effluents containing adsorbable organic halogens initiated a rapid development of non-chlorine containing alternatives for bleaching/delignification of chemical pulps. Such alternatives include oxygen ozone and hydrogen peroxide. By introduction of modified cooking processes and application of sequential bleachings using such oxygen compounds, it has been possible to achieve brightness levels of 85-87% ISO for soft wood kraft pulps.
A key factor in achieving feasible brightness levels and viscosities upon bleaching/delignification with peroxide, is pretreatment with a chelant ("Q-stage") prior to the peroxide bleaching ("P-stage"). This is a standard operation for removal of transition metal ions, in particular, manganese adsorbed to the fiber phase. Such extractions are typically carried out at a pH of 4.5 to 6. Manganese ions are not effectively chelated at pH's above 7, and therefore cannot be removed by dewatering and washing in a subsequent step. Alkaline extraction/washing is conventionally used in pulp making for achieving various characteristics of the pulp, but it has heretofore not been possible to combine it with an effective pretreatment of kraft pulps by chelants.
It is therefore an object of the present invention to provide a high-pH metal chelation process which for the pulp results in improved extraction of organic solvent extractives, improved washability of the pulp, and improved bleach response.
It is a further object of the present invention to provide a high-pH metal chelation process which for the pulp results in improved water absorption properties and improved taste and smell, particularly in the case of unbleached pulps.
It is a still further object of the present invention to provide a high-pH metal chelation process, which results in decreased formation of crusts in the production equipment.